Practical_Antenna_Handbook_0071639586

C h a p t e r 1 6 : M o b i l e a n d M a r i n e A n t e n n a s 381
Loading
coil
Adjustable
shaft
tenna are very popular on the 11-m citizens band, where they may be as short as 30 in
or as long as 48 in.
The other variant is shown in Fig. 16.2C. In this case, the lower end of the radiator
consists of a metal tube topped with a loading coil. An adjustable shaft at the top end is
used to tune the antenna to resonance. Commercially manufactured forms of this approach
are easily found; the New-Tronics Hustler product line, for instance, includes a
family of lower rods, couplers, mounts, and adapters, and features resonator coils for
both low and high power. The fixed lower rod is used on all bands, while the coil and
adjustable stinger form a separate “resonator” for each HF band. Multiband operation
of the Hustler can be accomplished by using a resonator mounting bracket such as the
three-way unit shown in Fig. 16.2D. (But don’t use it while moving; the stock Hustler
bracket is aluminum, which can quickly fatigue and crack when subjected to buffeting
while under way.) With no resonators at all atop it, the MO-1 or MO-2 fold-over shaft
can be used by itself on 6 m, especially if the mobile HF rig has an internal antenna
tuner (example: the Kenwood TS-480S/AT).
A common problem with all short coil-loaded mobile antennas is that they tend to
be very high Q antennas. In other words, they are very sharply tuned and have a very
narrow useable bandwidth; as little as 25-kHz change of operating frequency will detune
the antenna significantly on 80 m. Although an antenna tuner at the transmitter
will reduce the VSWR to a point that allows the transmitter
to operate, that type of tuner does not fully address the
problem. The actual problem is that the antenna is very short
relative to a quarter-wavelength, and the feedpoint impedance
of the antenna varies drastically with small changes in
operating frequency. The only cure for this problem is to readjust
the resonator’s adjustable shaft as the band segment
is changed. Unfortunately, this solution requires the opera-
Fixed
shaft
Resonator
#1
Resonator
#2
Resonator
#3
A
B Bracket Fixed shaft
Figure 16.2C Top-loaded mobile
antenna common in amateur
radio.
Figure 16.2D Multiband top-loaded antenna.